The present invention relates to apparatus for monitoring the inductance of a coil.
The present invention may be employed in apparatus for the electromagnetic control of the suspension of an object.
Electromagnetic control of the position of an object by suspension or levitation has been employed in a number of commercial applications in the field of industrial engineering. Such applications have included passenger-carrying vehicles, conveyor systems, flow meters, frictionless bearings, tool spindles, centrifuges, alternators, pumps, compressors and balances. One aspect of the present invention is concerned with systems for the electromagnetic control of the suspension of an object which are suitable for use in such applications and which systems are of the kind including an electromagnet comprising an inductive control coil, a source of electric power for the electromagnet which source includes a controllable electric supply device capable of delivering a controlled electric supply to the electromagnet, a control signal producer for generating an error control signal in response to an incremental change in a parameter related to the position of the object relative to the electromagnet and a negative feedback control loop for feeding an error control signal generated by the control signal producer to the electric supply device to adjust the electrical supply to the coil so as to stabilise the suspension of the object relative to the electromagnet.
The relative position of the object, ie the separation or gap between the electromagnet and the object being controlled, is monitored in the prior art by a transducer forming part of the control signal generator for the feedback loop. Such transducers have included devices which are photocells (detecting the interruption of a light beam by movement of the object); magnetic (comprising a gap flux density measurement device, eg Hall plate); inductive (employing two coils in a Maxwell bridge which is in balance when the inductance of the coils is equal); I/B detectors (in which the ratio of the electromagnet coil current and magnetic flux produced is determined to provide a measure of the gap between electromagnet and object; for small disturbances the division may be replaced by a subtraction); and capacitive (employing an oscillator circuit whose output frequency varies with suspension gap).
The use of the air gap or separation measurement transducers in prior systems has not been entirely satisfactory. Usually, the transducer has an upper temperature limit of operation. The transducers have a discrete physical size and the space occupied by the transducer reduces the space available for the electromagnet and therefore the force which can be exerted on the object.
This problem is significant where the object is a shaft which is able to bend, eg at a resonant rotation frequency in its so-called "free-free" mode, confining the positions in which the transducer may be located.